Damper assemblies having controlled valves that are associated with their piston components, and that may be used to change the damping coefficients of the dampers, are known: see, e.g., U.S. Pat. Nos. 4,635,765, 4,620,619, 4,463,839, 3,420,341, 3,063,518, 1,205,433, and UK application publication No. 2,164,120A. When employed in the suspension of an automobile or similar vehicle, such dampers permit the "stiffness" of the suspension to be adjusted for different driving conditions. For example, if the vehicle is traveling at high speed and/or over a relatively rough road surface, the damping coefficient of the assemblies may be increased so as to provide increased vehicle stability and passenger comfort. Similarly, if the vehicle is traveling over a relatively smooth road surface at a relatively low speed, decrease of the damping coefficient of the assemblies might be desirable and could be effected.
The foregoing and many other conventional uses of controllable damper assemblies do not require especially rapid or frequent operation of their valve components. However, rapid and frequent valve operation is required in certain utilizations, such as when a damper of the type in question is to be employed for implementation of a vibration control policy similar to that disclosed in U.S. Pat. Nos. 3,807,678 and 4,491,207. The valve in the piston of a damper used for such purpose must be capable of very rapid and substantially continuous operation over extended periods of time even when there is a large pressure differential across the piston. When the valve is in its fully open position, it additionally should then permit a large flow in either direction through the piston. The valve desirably should also be operable by inexpensive means, such as a single or double acting linear actuator of the solenoid or other common type.